1. Field of the Invention
This invention relates to a shadow mask for a color cathode-ray tube (C-CRT) and more particularly to prevention of thermal strain of the shadow mask.
2. Description of the Prior Art
Referring to FIG. 1, there is schematically shown an exploded perspective view of a C-CRT. A glass enclosure of a C-CRT comprises a panel portion 10 and a funnel portion 12. An electron gun 11 is provided inside a neck portion of the funnel 12. The panel 10 is to be welded to the funnel 12 with glass frit, enclosing a shadow mask 13 which is fixed to the panel 10 by support members.
A method for fabricating a shadow mask is described in Electronic Science, 1964, Vol. 14, No. 9, pp. 36 and 39-40 published by Sanpou Inc. of Japan.
Referring to FIG. 2, there is shown an enlarged fragmentary sectional view of a basic plate in a conventional shadow mask. A shadow mask 13 comprises a basic iron plate 14 of 0.15-0.25 mm thickness which has a plurality of apertures 15 for electron beams. Each of those apertures is generally circular or rectangular in shape.
At first, the basic iron plate is flat in shape and is subjected to a heat treatment (at 700.degree.-920.degree. C. in an atmosphere of hydrogen). The iron plate is then pressed to conform it to the spherical shape of the inner surface of the panel 10. Thereafter, the pressed plate is subjected to a surface treatment for coating the plate with black rust. This surface treatment is called "blackening process". In the blackening process, the iron plate is immersed into an alkaline solution containing an oxidizing agent. Alternatively, the black rust coating can also be obtained using steam or carbon oxide gas. The black rust coating thus obtained prevents red rust on the basic plate when the shadow mask is heated to about 400.degree. C. in the air during the assembling process of the C-CRT.
In the meantime, about 80% of electrons emitted from the gun 11 impinge upon the shadow mask 13 and kinetic energy of those electrons is converted into heat energy. Due to the heat thus generated, the shadow mask expands thermally and causes a thermal strain. The above described black rust coating increases the thermal emissivity of the shadow mask to about 0.75 and thus reduces the thermal strain of the shadow mask. In view of reproduction of images with high fidelity in recent C-CRTs, it is desirable that the panel face is made flat, that the density of picture elements is increased and that the images are brightened and made more clear. To satisfy these demand, it is required that the size of apertures for electron beams and the pitch of those apertures both need to be small. The smallest size of the apertures depends on the thickness of the basic plate for a reason of processing technique. To form apertures 0.150 mm in diameter, it is necessary that the basic plate is as thin as 0.150 mm in thickness.
The conversion ratio of kinetic energy of electron beams into thermal energy depends on the thickness of the shadow mask. When the basic plate is thin, the above described black rust (Fe.sub.3 O.sub.4) coating with the emissivity of about 0.75 is not enough to prevent the thermal strain of the shadow mask 13.